Chapter 8

1. Chapter 8 An Introduction to Metabolism

2. Overview: The Energy of Life • The living cell • Is a miniature factory where thousands of reactions occur • Converts energy in many ways

3. Figure 8.1 • Some organisms • Convert energy to light, as in bioluminescence

4. Concept 8.1: An organism’s metabolism transforms matter and energy, subject to the laws of thermodynamics

5. Metabolism • Is the totality of an organism’s chemical reactions • Arises from interactions between molecules

6. Enzyme 1 Enzyme 2 Enzyme 3 A D C B Reaction 1 Reaction 2 Reaction 3 Startingmolecule Product Organization of the Chemistry of Life into Metabolic Pathways • A metabolic pathway has many steps • That begin with a specific molecule and end with a product • That are each catalyzed by a specific enzyme

7. Catabolic pathways • Break down complex molecules into simpler compounds • Release energy

8. Anabolic pathways • Build complicated molecules from simpler ones • Consume energy

9. Forms of Energy • Energy • Is the capacity to cause change • Exists in various forms, of which some can perform work

10. Kinetic energy • Is the energy associated with motion • Potential energy • Is stored in the location of matter • Includes chemical energy stored in molecular structure

11. On the platform, a diver has more potential energy. Diving converts potential energy to kinetic energy. Climbing up converts kinetic energy of muscle movement to potential energy. In the water, a diver has less potential energy. Figure 8.2 • Energy can be converted • From one form to another

12. The Laws of Energy Transformation • Thermodynamics • Is the study of energy transformations

13. The First Law of Thermodynamics • According to the first law of thermodynamics • Energy can be transferred and transformed • Energy cannot be created or destroyed

14. Chemical energy (a) First law of thermodynamics: Energy can be transferred or transformed but Neither created nor destroyed. For example, the chemical (potential) energy in food will be converted to the kinetic energy of the cheetah’s movement in (b). Figure 8.3  • An example of energy conversion

15. Heat co2 + H2O Second law of thermodynamics: Every energy transfer or transformation increases the disorder (entropy) of the universe. For example, disorder is added to the cheetah’s surroundings in the form of heat and the small molecules that are the by-products of metabolism. (b) Figure 8.3  The Second Law of Thermodynamics • According to the second law of thermodynamics • Spontaneous changes that do not require outside energy increase the entropy, or disorder, of the universe

16. 50µm Figure 8.4 Biological Order and Disorder • Living systems • Increase the entropy of the universe • Use energy to maintain order

17. Concept 8.2: The free-energy change of a reaction tells us whether the reaction occurs spontaneously

18. Free-Energy Change, G • A living system’s free energy • Is energy that can do work under cellular conditions

19. The change in free energy, ∆Gduring a biological process • Is related directly to the enthalpy change (∆H) and the change in entropy • ∆G = ∆H – T∆S

20. Free Energy, Stability, and Equilibrium • Organisms live at the expense of free energy • During a spontaneous change • Free energy decreases and the stability of a system increases

21. More free energy (higher G) • Less stable • Greater work capacity • In a spontaneously change • The free energy of the system decreases (∆G<0) • The system becomes more stable • The released free energy can • be harnessed to do work . • Less free energy (lower G) • More stable • Less work capacity (a) Gravitational motion. Objects move spontaneously from a higher altitude to a lower one. (c) (b) Diffusion. Molecules in a drop of dye diffuse until they are randomly dispersed. Chemical reaction. In a cell, a sugar molecule is broken down into simpler molecules. Figure 8.5  • At maximum stability • The system is at equilibrium

22. Free Energy and Metabolism

23. Reactants Amount of energy released (∆G <0) Free energy Energy Products Progress of the reaction Figure 8.6 (a) Exergonic reaction: energy released Exergonic and Endergonic Reactions in Metabolism • An exergonic reaction • Proceeds with a net release of free energy and is spontaneous

24. Products Amount of energy released (∆G>0) Free energy Energy Reactants Progress of the reaction Figure 8.6 (b) Endergonic reaction: energy required • An endergonic reaction • Is one that absorbs free energy from its surroundings and is nonspontaneous

25. ∆G < 0 ∆G = 0 (a) A closed hydroelectric system. Water flowing downhill turns a turbine that drives a generator providing electricity to a light bulb, but only until the system reaches equilibrium. Figure 8.7 A Equilibrium and Metabolism • Reactions in a closed system • Eventually reach equilibrium

26. (b) An open hydroelectric system. Flowing water keeps driving the generator because intake and outflow of water keep the system from reaching equlibrium. ∆G < 0 Figure 8.7 • Cells in our body • Experience a constant flow of materials in and out, preventing metabolic pathways from reaching equilibrium

27. ∆G < 0 ∆G < 0 ∆G < 0 (c) A multistep open hydroelectric system. Cellular respiration is analogous to this system: Glucoce is brocken down in a series of exergonic reactions that power the work of the cell. The product of each reaction becomes the reactant for the next, so no reaction reaches equilibrium. Figure 8.7 • An analogy for cellular respiration

28. Concept 8.3: ATP powers cellular work by coupling exergonic reactions to endergonic reactions • A cell does three main kinds of work • Mechanical • Transport • Chemical

29. Energy coupling • Is a key feature in the way cells manage their energy resources to do this work

30. Adenine NH2 C N C N HC O O O CH C N - N O O O O CH2 O - - - O O O H H Phosphate groups H H Ribose Figure 8.8 OH OH The Structure and Hydrolysis of ATP • ATP (adenosine triphosphate) • Is the cell’s energy shuttle • Provides energy for cellular functions

31. P P P Adenosine triphosphate (ATP) H2O Energy + P i P P Adenosine diphosphate (ADP) Inorganic phosphate Figure 8.9 • Energy is released from ATP • When the terminal phosphate bond is broken

32. Endergonic reaction: ∆G is positive, reaction is not spontaneous NH2 NH3 + ∆G = +3.4 kcal/mol Glu Glu Glutamine Glutamic acid Ammonia Exergonic reaction: ∆ G is negative, reaction is spontaneous ∆G = + 7.3 kcal/mol + P ADP H2O ATP + Coupled reactions: Overall ∆G is negative; together, reactions are spontaneous ∆G = –3.9 kcal/mol Figure 8.10 • ATP hydrolysis • Can be coupled to other reactions

33. How ATP Performs Work • ATP drives endergonic reactions • By phosphorylation, transferring a phosphate to other molecules

34. P i P Motor protein Protein moved (a) Mechanical work: ATP phosphorylates motor proteins Membrane protein ADP + ATP P i P P i Solute Solute transported (b) Transport work: ATP phosphorylates transport proteins P NH2 + + NH3 P i Glu Glu Reactants: Glutamic acid and ammonia Product (glutamine) made Figure 8.11 (c) Chemical work: ATP phosphorylates key reactants • The three types of cellular work • Are powered by the hydrolysis of ATP

35. ATP hydrolysis to ADP + P i yields energy ATP synthesis from ADP + P i requires energy ATP Energy from catabolism (exergonic, energy yielding processes) Energy for cellular work (endergonic, energy- consuming processes) ADP + P i Figure 8.12 The Regeneration of ATP • Catabolic pathways • Drive the regeneration of ATP from ADP and phosphate

36. Concept 8.4: Enzymes speed up metabolic reactions by lowering energy barriers • A catalyst • Is a chemical agent that speeds up a reaction without being consumed by the reaction

37. An enzyme • Is a catalytic protein

38. The Activation Barrier • Every chemical reaction between molecules • Involves both bond breaking and bond forming

39. CH2OH CH2OH CH2OH CH2OH O O O O H H H H H H H Sucrase H OH H HO OH H HO H2O O + H H OH O HO CH2OH CH2OH OH H H H OH H OH OH Fructose Glucose Sucrose Figure 8.13 C12H22O11 C6H12O6 C6H12O6 • The hydrolysis • Is an example of a chemical reaction

40. The activation energy, EA • Is the initial amount of energy needed to start a chemical reaction • Is often supplied in the form of heat from the surroundings in a system

41. A B D C Transition state B A EA D C Free energy Reactants B A ∆G < O C D Products Progress of the reaction Figure 8.14 • The energy profile for an exergonic reaction

42. How Enzymes Lower the EA Barrier • An enzyme catalyzes reactions • By lowering the EA barrier

43. Course of reaction without enzyme EA without enzyme EA with enzyme is lower Reactants Free energy ∆G is unaffected by enzyme Course of reaction with enzyme Products Progress of the reaction Figure 8.15 • The effect of enzymes on reaction rate

44. Substrate Specificity of Enzymes • The substrate • Is the reactant an enzyme acts on • The enzyme • Binds to its substrate, forming an enzyme-substrate complex

45. Substate Active site Enzyme Figure 8.16 (a) • The active site • Is the region on the enzyme where the substrate binds

46. Enzyme- substrate complex (b) Figure 8.16 • Induced fit of a substrate • Brings chemical groups of the active site into positions that enhance their ability to catalyze the chemical reaction

47. Catalysis in the Enzyme’s Active Site • In an enzymatic reaction • The substrate binds to the active site

48. 1 Substrates enter active site; enzyme changes shape so its active site embraces the substrates (induced fit). 2 Substrates held in active site by weak interactions, such as hydrogen bonds and ionic bonds. 3 Active site (and R groups of its amino acids) can lower EA and speed up a reaction by • acting as a template for substrate orientation, • stressing the substrates and stabilizing the transition state, • providing a favorable microenvironment, • participating directly in the catalytic reaction. Substrates Enzyme-substrate complex 6 Active site Is available for two new substrate Mole. Enzyme 5 Products are Released. 4 Substrates are Converted into Products. Figure 8.17 Products • The catalytic cycle of an enzyme

49. The active site can lower an EA barrier by • Orienting substrates correctly • Straining substrate bonds • Providing a favorable microenvironment • Covalently bonding to the substrate

50. Effects of Local Conditions on Enzyme Activity • The activity of an enzyme • Is affected by general environmental factors